Hitherto, there is a dynamic microphone in which a voice coil attached to a diaphragm which vibrates by a sound wave which is emitted from a sound source is vibrated integratedly with the diaphragm in a gap of a magnetic circuit and a moving speed of the voice coil is output as an electric signal. Such a dynamic microphone is widely used for business and at ordinary homes.
An example of the dynamic microphone which is generally used is shown in FIGS. 4 to 7. FIG. 4 is a cross sectional view showing an outline of the dynamic microphone. The dynamic microphone mainly comprises: a diaphragm 1; a magnet 2; a voice coil 3; and a casing 4 to which the peripheral edge portion of the diaphragm 1 is joined and which has therein various component elements of the microphone. That is, the cylindrical voice coil 3 is located in a narrow gap G between the outer peripheral surface of a pole piece 5 and the inner peripheral surface of a yoke plate 6. The pole piece 5 is made of magnetic soft iron so as to have a disk-like shape and is joined to the front surface of the magnet 2. The yoke plate 6 is likewise made of magnetic soft iron so as to have an almost pan-like shape and is joined to the back surface of the magnet 2. The tip of the voice coil 3 is fixed to the center portion of the diaphragm 1, that is, the outer peripheral portion of a center dome 1a of the diaphragm 1. A peripheral edge portion 1c as an outer peripheral edge of an edge portion 1b locating in the outer peripheral portion of the center dome 1a of the diaphragm 1 is attached to the outer edge portion of the front surface of the casing 4 provided in the outer peripheral portion of the yoke plate 6 by an adhesive agent 7. The gap G in which the voice coil 3 is located constructs the magnetic circuit together with the pole piece 5, yoke plate 6, and magnet 2. When the diaphragm 1 vibrates by a sound wave from the sound source, the voice coil 3 and the diaphragm 1 integratedly vibrate in the gap G. A current flows in the voice coil 3 in accordance with a deviation by the vibration. By detecting and amplifying the current, a voice signal is obtained. In the diagram, reference numeral 8 denotes a through hole which penetrates the inside and outside of the casing 4. Reference numeral 9 denotes an elastic member arranged in contact with the through hole 8.
For the diaphragm 1 of the dynamic microphone which has schematically been constructed as mentioned above, it is required to set a low band limit to a low frequency. For this purpose, it is necessary to set a resonance frequency to a low value. To set the resonance frequency to a low value, there can be mentioned methods such that a weight of the voice coil 3 is increased, a material of the diaphragm 1 is made thin, the shape of the edge portion 1b of the diaphragm 1 is changed so as to have a low resonance frequency, and the like.
However, when the weight of the voice coil 3 is increased, the vibration noises are increased and the working efficiency is deteriorated. On the other hand, if the material itself of the diaphragm 1 is made thin or the shape of the edge portion lb is changed, it causes an abnormal resonance in a middle high frequency range. Although such an abnormal resonance can be reduced to a certain degree by the shape of the diaphragm 1 or the like, there is a large experimental element when determining such a shape. Further, the costs of trial manufacture are also high and the costs eventually rise.
It is considered that causes of the abnormal resonance depend on not only the shape of the edge portion 1b but also the adhesive characteristics of the adhesive agent 7. That is, as shown in FIG. 5, the peripheral edge portion 1c of the edge portion 1b of the diaphragm 1 is strictly joined to the casing 4 in a state in which the adhesive agent 7 is swollen in the inside of the edge portion 1b. When the adhesive agent 7 is hardened, the stiffness of the diaphragm 1 is raised due to a coating amount of the adhesive agent or a difference of the wettability, or by changing the characteristic frequency, the resonance is caused at an unintended frequency. In the case where the adhesive agent 7 was hardened in the peripheral edge portion 1c of the edge portion 1b in a state in which the adhesive agent 7 is not swollen in the inside of the edge portion 1b, as shown in a frequency response characteristic diagram of FIG. 6, it will be obviously understood that a dip of about 8 dB occurs in the characteristics near 5 kHz in such an abnormal resonance. In the case where the adhesive agent 7 was hardened in a state in which the adhesive agent is swollen in the inside of the edge portion 1b from the peripheral edge portion 1c, as shown in a frequency response characteristic diagram of FIG. 7, it will be obviously understood that such a dip of about 8dB also occurs in the 0 characteristics near 5 kHz. Therefore, the adhesive agent 7 to fix the peripheral edge portion 1c of the edge portion 1b to the casing 4 or the adhering structure by the adhesive agent 7 causes a problem.